Method of improving the magnetostriction and core loss of cube-on-face oriented magnetic steels



United States Patent 3,533,861 METHOD OF IMPROVING THE MAGNETOSTRIC-TION AND CORE LOSS OF CUBE-ON-FACE ORIENTEDMAGNETIC STEELS Karl Fosterand Joseph Seidel, Pittsburgh, Pa., assignors to Westinghouse ElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.Continuation-in-part of application Ser. No. 576,963, Sept. 2, 1966,which is a continuation-in-part of application Ser. No. 556,337, June 9,1966. This application Apr. 10, 1968, Ser. No. 720,333

Int. Cl. H01f 1/18, 27/24; B3211 15/00 US. Cl. 148-113 11 ClaimsABSTRACT OF THE DISCLOSURE The method is set forth for improving themagnetostriction and core loss of cube-on-face oriented silicon steels.A glass coating is fused to the steel while the same is under tension.Data is included illustrating magnetostriction, core loss and othermagnetic data.

CROSS REFERENCE TO RELATED APPLICATION This invention is acontinuation-in-part of our copending application Ser. No. 576,963,filed Sept. 2, 1966 in the names of Karl Foster and Joseph Seidel andentitled Glass Coated Electrical Steel Sheet and Articles PreparedTherefrom, which is a continuation-in-part of application Ser. No.556,337, filed June 9, 1966 in the names of Karl Foster and JosephSeidel, now abandoned.

BACKGROUND OF THE INVENTION In our copending application above referredto it was shown that the application of glass alone to the cube-onedgeoriented silicon steels resulted in very low magnetostriction values inthe rolling direction. However, when the same glasses were applied tosilicon steel characterized by the cube-on-face orientation, values aslow as those for cube-on-edge oriented silicon steels were not obtained,although some reduction in magnetostriction was observed.

Part of the reluctance to the commercial acceptance of cube-on-facematerial resides in the fact that the core loss exhibited by thecube-on-face steels was at a higher level than that for cube-on-edgematerials although the so-called corner loss in a power transformerapplication was considerably less because of the bilateral orientation.Moreover, as a class, the cube-on-face materials exhibit a significantlyhigher magnetostriction than that of cube-on-edge materials.

It has been postulated that the dilference in response of themagnetostriction properties to the glass coating of cube-on-edgeoriented silicon steel and cube-on-face oriented silicon steels is aresult of variation of the mechanical properties with respect to theorientation of each class of materials. For example, Youngs modulus forsilicon iron is much lower in the [100] direction than in either the[110] or [111] directions. In cube-on-edge oriented silicon steels whichhas only one [100'] direction, that is in the direction of rolling,there results a significant tensile strain only in the direction ofrolling when a glass having a low thermal expansion is applied, therebyresultig in a large effect on the magnetostriction in thisparticular'direction. On the other hand, cube-onface oriented siliconsteel ideally has two directions of easiest magnetization, one being inthe rolling direction and the other being in the same plane butperpendicular to the rolling direction. As a result, the application ofglass to cnbe-on-face oriented silicon steels must then tend to createan equal tensile strain in both directions.

Patented Oct. 13, 1970 ice However, the strain cannot be as large in anyone direction as that for cube-on-edge oriented silicon steel in therolling direction, nor the effect on the magnetostriction I sopronounced. Consequently, an additional means must SUMMARY OF THEINVENTION The present invention utilizes a cube-on-face oriented siliconsteel containing up to 5% silicon, and preferably from about 2% to about4% silicon,.and other minor components as will be set forth hereinafter.The steel is first treated to produce an adherent coating on the surfacethereof since in the production of cube-on-face oriented silicon steel,the surfaces are clean to the extent of being completely oxide free,resulting from the last anneal which develops the cube-on-ifaceorientation. The steel is thereafter subjected to the application of aglass coating thereto which coating has a characteristic low coefficientof thermal expansion, which coefficient must be lower than that of thesteel. Thereafter the coating is fused and during such fusion the steelis subjected to a predetermined amount of tension preferably in therolling direction. Upon fusion of the glass coating the steel ispermitted to cool to a temperature below the softening point of theglass while maintaining the steel in tension.

The-above process is effective for producing low values ofmagnetostriction preferably in the rolling direction which isaccomplished with a corresponding improvement in the core loss exhibitedby the silicon steel.

It is an object of the present invention to provide a process forimproving the magnetostriction of silicon steel characterized bycube-on-face orientation.

It is another object of the present invention to provide a process forapplying glass to the surface of cubeon-face oriented silicon steel forimproving the magnetostriction and the core loss exhibited by thesilicon steel.

Other objects of the present invention will become apparent to thoseskilled in the art when taken in conjunction with the followingdescription.

DESCRIPTION OF THE PREFERRED EMBODIMENT The steel to which the processof the present invention is applicable is a silicon iron which ischaracterized by a silicon content of up to about 6% and preferablybetween about 2% and 4% silicon, and small amounts of up to 4% ofchromium, up to 0.4% manganese, up to 1% nickel,

and up to 0.2% molybdenum. Outstanding results have 7 their last annealwith a clean surface, it becomes incumbent to provide an intermediatelayer Well bonded to the surface of the silicon steel for the properadherence of the glass thereto. This may be accomplished in a number ofways. One expedient is to anneal the cube-on-face oriented materials ata temperature of about 1200 C. in the presence of MgO which ispreferably applied to the steel in the form of a thin slurry or aqueoussuspension coating. Such anneal is preferably performed employing ahydrogen atmosphere. The intermediate layer so formed com prising a wellbonded ferrous magnesium silicate provides adherence of the glass to thesteel and is more clearly described in our copending applicationreferred to hereinbefore. Alternatively,' this intermediate layer may beformed in any number of ways and contains any number of components,examples of which arenickel plating, vapor deposited aluminum or a heatreacted magnesium phosphate slurry. It is sufiicient so that theintermediate layer provides for good adherence of the glass to the layerand said intermediate layer of course must be integrally bonded to thesilicon steel. Ordinarily, such intermediate layer is of a thickness ofthe order of 0.05 to 0.3 mil.

With the properly prepared surface of the silicon steel there is appliedthereto a glass frit which is characterized by possessing a thermalexpansion of less than that of the steel and outstanding results havebeen obtained where the glass exhibits a coefficient of thermalexpansion of less than about 8 X10" in./ in. C. In addition it is alsopreferred to have a glass which will have a fusion point of not greaterthan about 950. The glass frit is applied to the steel in any convenientmanner, for example, by dipping or roll coating. While the thickness isnot critical nonetheless it is desired to maintain the thickness of theglass coating as small as possible in order to obtain the highest spacefactor. The thinness of this coating will prevent undue flaking orspalling in the event of reasonable bending or fiexure of the siliconsteel after the glass has been fused. Typically, the thickness of about0.1 to about 0.3 mil per side is desired. Next, the steel with the glassfrit in place is fired preferably at a temperature not in excess of 950C. During this firing, the glass frit will react to fuse and to providea tightly adherent layer of glass to the intermediate layer formed onthe silicon steel. During such fusion or while the temperature of thisfiring is maintained above the softening point of the glass, the

point of the glass, usually about 400 C. is a safe temperature for mostusable glasses. During this time the stresses are maintained and areonly discontinued when a conveni ent temperature is obtained below thesoftening point of the glass. The process of the present invention isadaptable for both batch as well as continuous type operations.

EXAMPLE I A 3% silicon steel having over 90% of its volume with the(100) plane within 10 of the sheet direction but with only about 50% ofits volume with a [100] direction within 10% of the rolling directionexhibited an induction at 10 oersteds of about 17,300 gausses. Thissteel, referred to as a low permeability steel, was annealed at 1200 C.in a hydrogen atmosphere with a thin aqueous MgO coating applied to thesteel to form an intermediate ferrous magnesium silicate layer less than0.1 mil thick thereon for adherence of the glass. Thereafter a glassfrit was applied to the steel and this glass frit has a thicknessv ofabout 0.25 mil per side. The glass, identified as LM103 had acomposition which included 38.0% PbO, 27.0% SiO 10.0% Na O, 9.5% B 06.0% MgO, 3.5% BaO, 2.0% K 0, 2.0% Li O and 1.0% CaO. Its coefficient ofexpansion is about 13 10 in./in. C. The glass coating was first firedwithout stress at a temperature between 750 and 800 C. The samples werethen reheated to between 500 and 550C. which is above the softeningtemperature of the glass and cooled under variously applied tensileloads.

Reference is directed to Table I which illustrates the magnetic testresults for the materials under the conditions stated.

TAB LE I.MA GNETIC PROPE RTIES Core Losses Magnetostriction (X10 steelmust be subjected to a tensile stress of at least 50 p.s.i. and up toabout 2000 p.s.i. This stressing is done preferably in the rollingdirection in order to apply additional stresses thereto as will be morefully explained hereinafter.

As will appear more clearly hereinafter, any amount of stress willimprove the magnetostriction and the core loss exhibited by the steel.However, it will be appreciated that when the steel is heated to atemperature near 950 C. the maximum stress which can be safely appliedto the steel is about 350 p.s.i. The application of higher stress willcause plastic deformation which may partially or completely destroy thecube-on-face orientation. Consequently, the applied stress shoulddecrease with corresponding increases of temperatures above thesoftening point of the glass. Where, however, the glass is fired nearits softening point it may be advantageous to apply as much as 2000p.s.i. stress in order to obtain the advantages of this invention.

Further, it is preferred to vary the stress to be applied to the steelin accordance with the permeability of the steel. More specifically, ithas been found that as the permeability of the steel increases the givenstress for a given temperature should also increase. Thus, for a lowpermeability steel stressed 350 p.s.i. in the rolling direction it willrequire a higher stress in the rolling direction at the same temperaturefor a steel exhibiting a higher permeability in order to obtain the samedegree of improvement in magnetostriction and core loss. This isbelieved to be partially explained by reason of the fact that the higherpermeability is associated with a greater degree of orientation. Afterthe glass has fused and while the steel is maintained in the stressedcondition it is cooled until the temperature of this steel and glassdrops below the softening It is noted from Table I that without glassthe material exhibited a core loss of .81 watt per pound and at 15 kg. amagnetostriction of 12.3 10 Merely applying the glass to the strip didnot in and of itself produce any sufficient difference in themagnetostriction nor in the core loss properties. However, when thesteel was subjected to the conditions of stress set forth, that is 350p.s.i. and 700 p.s.i. while above the softening temperature of theglass, and the material was cooled under those conditions of stresswhile being cooled until the softening point of the glass was passed, itis noted that while there is great improvement in the magnetostrictionexhibited by the steel resulting from the application of stress,nonetheless the level of these values are considered to be clearly high.Table I also records a slight improvement in core loss. It is believedthat such modest improvements arose since the glass employed, that isLM103, had a coefiicient of thermal expansion which rather closelymatched that of the steel, and no additional tensile stresses wereimparted to the steel resulting from the fusion of the glass and thecooling to room temperature. Consequently, when the glass has acoefficient of thermal expansion closely approximating that of the steelthere will appear some improvement in magnetostriction and core loss butthe degree of improvement is small in comparison to that exhibited bythe steel when a glass is employed having a coefficient of less thanabout 8.0x 10*.

EXAMPLE II The same steel as set forth in Example I was coated with aglass identified as PH115. This glass had a composition of 66.0% P 012.0% MnO, 10.5% A1 0 5.5% ZnO, 2.75% SiO 1.5% Na O, 0.75% CaO, 0.5%AS203, and 0.5 V 0 and a coefficient of thermal expansion of 7.2 in./in. C. As a result of the difference in the coefficient of thermalexpansion between the steel and the glass substantial tensile stressesare imparted to the coated sheet upon cooling to room temperature. Thesteel having a coating thickness of about 0.5 mil total was fired at atemperature of 750 and 800 C. to fuse the glass. The samples werereheated to between 500 and 550 C., which exceeded the softening pointof the glass, and cooled under the applied tensile loads as set forthhereinafter in Table TABLE II.MAGNETIC PROPE RTIES Core LossesMagnetostrietion (X10 Stress B.- B Pc /60 Pc 17/60 Glass (p.s.i (0e(gauss) (gauss) (w./lb.) (w./lb.) 15 kg. 17.5 kg. 20 kg.

None 070 10, 000 17, 300 81 1. 21 12. 3 PH115 079 9, 100 17. 200 82 1.28 3. 3 5, 9 7 9 PH115 350 062 10, 500 17, 600 75 l. 07 2. 4 2. 4 -3. 5700 063 9, 400 17, 300 .75 1.10 -1. 0 -o. s 1, 1

From the test results set forth in Table II it is apparent that the mereapplication of the glass to the steel alone produces a significantdecrease in the magnetostriction. This was expected because of thesignificantly lower coefficient of thermal expansion exhibited by theglass PH115 in comparison with steel. Where, however, the steel wassubjected to the tensile load of 350 p.s.i. and 700 p.s.i., it is notedthat a further greatly pronounced, decrease in the magnetostriction isobtained. The improvement in the magnetostriction is also accompanied byan improvement in the core loss exhibited by these materials. While thechange in the remanence characteristics are not significant, thecoercive force and induction at B do not deteriorate. Thus theapplication of the stress together with the selection of the properglass provides for outstanding improvement in magnetostrictionproperties as well as reduced core losses without significantdeterioration in the balance of the magnetic characteristic.

EXAMPLE III Another cube-on-face oriented 3 /2% silicon steel wasselected and this steel exhibited over 80% of its volume beingcube-on-face grains having cube edges with the [100] direction within 10of the rolling direction. Consequently, this steel, which had aninduction of about 18,600 gausses at 10 oersteds, is referred to as ahigh permeability steel. This steel was annealed at 1200 C. in ahydrogen atmosphere and the surfaces were coated with a thin layer of anaqueous MgO slurry to provide a thin intermediate layer for adherence ofthe glass, the same as in Examples I and II. Thereafter, a coating ofglass PHllS to a total thickness of 0.5 mil was again applied to thissteel which was first fired without stress at a temperature between 750and 800 C. Thereafter the samples were reheated to a temperature withinthe range between 500 and 550 C. and a load applied as set forthhereinafter. The steel was cooled to a temperature below the softeningpoint of the glass while maintaining the tensile stress in the rollingdirection as set forth more clearly hereinafter in Table HI.

However when the steel was cooled under an applied load of 350 poundsper square inch until the softening point of the glass was surpassed, amore significant improvement both in the magnetostriction and in thecore losses is exhibited by the steel as processed in accordance withthe teachings of this invention.

From the foregoing it has been clearly demonstrated that a number ofcriteria must be employed in order to produce the outstanding results asdemonstrated hereinbefore. These criteria include the selection of theglass having a sufficiently low coefficient of thermal expansion andpreferably a coeflicient of thermal expansion of less than 8 l0 in./in.C. In addition thereto it is also necessary to maintain the steel understress during cooling from the glass fusion temperature until the steelreaches a temperature below the softening point of the glass. Duringthis time the load to be applied should be a load above 50 p.s.i. andpreferably between about 350 p.s.i. and 2000 p.s.1.

Care must be exercised however so as not to apply too great a load forit is possible to induce suflicient strain and/or creep to the steel soas to deteriorate the observable magnetic characteristics. Accordingly,it is believed that about 2000 pounds per square inch at the elevatedtemperature should be about the upper level to which the steel isstressed. The steel stressed to less than about 350 p.s.i. attemperatures approaching the softening point of the glass did not appearto significantly change the magnetostriction over that produced byapplying to the steel a glass having the proper coefiicient of thermalexpansion to thereby induce the tensile stresses as a result of thedifferences in the coefiicient of thermal expansion between thatexhibited by the glass and that exhibited by the steel. 'However, theselower stresses are effective where the softening point of the glass isquite high.

Characteristically, while a glass of the phosphate type has proved to beeffective, other glasses can be employed so long as they produce acoefiicient of thermal expansion of about less than 8X l0 in./in. C.Success has been TABLE II.MA GNETIG PROPE RTIES Core LossesMagnetostriction (X10 Stress H, Br Bm Pc 15/60 Pc 17/60 Glass (p.s.i.)(0a.) (gauss) (gauss) (w./lb.) (w./lb.) 15 kg. 17.5 kg. 20 kg had withglasses having the following range of composisquare inch during coolingof the glass to below the tions. softening point thereof.

4. .The method of claim 1 in which the glass has a Glass Glasscomposition of: weight percent weight percent r C fluent Weight pm centons 1 COIIISPtIlJUBUlZ. 60 P205 60 70 MnO 10-14 A1 0 9-12 ZnO 4-7 SiO1.5-3.5 Na O 0-2 CaO 0-1 AS203 0-1 1 Principally balance. V205 0-1 Withthe glass having a thermal expansion of less than about 8X10 in./ in.C., the glass should also exhibit. g gg g of claim 1 m whlch the glasshas a com a fusion point of less than about 950 C. This is desirablesince use of glasses melting at higher temperatures may Constitutent:Weight percent produce difficulties through excessive oxidation or theyZnO 31-60 detrimentally alfect the magnetic characteristics exhibitedSiO 8 14 by the steel. Consequently, it is desired to provide the PhD10-44 glass frit with a fusion temperature not in excess of 950 B 011-22 C. and preferably one between 750 and 800 C. A1 0 Principallybalance It should be noted that in each of the foregoing examples theglass frit Was first fused and thereafter I'e- The method of Claim 1 1hWhleh the glass exhlblts a heated and cooled under the applied tensileloads. It is temperature of less than about apparent that the processmay be carried out in a single 7. The method of claim 1 in which thestress applied heating step, wherein the steel is subjected to thetension to the steel varies inversely with the temperature to whichduring the fusion and cooled directly therefrom without the steel isheated. the necessity for any reheating. It is clearly contemplated 8.The method of claim 1 in which the stress applied that when the steel isprocessed on a continuous basis to the steel at a given temperatureabove the softening the frit will be applied to the silicon steel andfired at a point of the glass varies directly with the permeabilitysufficiently high temperature to fuse the glass and the of the steel.

Cooling below the Softening Point of the glass will take 9. In theprocess for improving the magnetostriction in P e While h p maintained za Continuous the rolling direction and the core loss of cube-on-facetenslle load Wlthlh the hmlts Set forth herelnbefofe. oriented siliconsteels containing from about 2% to about Thus the continuous strip canbe passed into a long 4% silicon by applying a glass coating thereto andin vertical furnace having a heating section at the bottom which thesteel has been subjected to the formation of a while the cooling takesplace in the upper part, the strip thin intermediate layer for theadherence of glass thereto, being under a suitable tension between upperand lower the steps comprising, applying a glass coating to the steelrolls at the upper and lower ends of the furnace with which coating ischaracterized by a fusion temperature of the weight of the stripapplying a higher load on the less than about 950 C. and a coefficientof thermal excooling portion of the strip than is present in the lowerpansion of less than 8 10- in./in. C., firing said glass portion of thevertical furnace where the glass is being coating to fuse the same tothe intermediate layer on fused. Thus in a 100 foot vertical length, theload at the the steel, subjecting said steel to a tensile stress of upto upper end is about 300 p.s.i. due to the weight of the 2000 psi. inthe rolling direction and cooling said steel strip above. whilemaintaining said steel under tension until the tem- It will beunderstood by those skilled in the art that perature of the glass isbelow the softening point. although the invention has been described inconnection 10 h process f claim 9 in which the applied tensile withpreferred embodiments, modification and variations stress i i dinversely Within the range f up to 2000 in the Processing Schedule andin other aspects of the p.s.i. with the temperature to which the steelis heated vention may be employed without departing from the above theft i point of the glass. underlying'spirit and scope of the inventionset forth in 11, Th process of l i 9 i hi h th li d t the ppclttllmssile stress is varied directly within the range of up to 2000 Weclaim as our invention: p.s.i. with the permeability exhibited by thesteel. 1. In the method of improving the magnetostriction and core lossof cube-on-face oriented steels containing References Cited up to about5% silicon, comprising up to 4% chromium, UNITED STATES PATENTS up to0.4% manganese, up to 1% nickel and up to 0.2% molybdenum, the balancebeing iron and impurities, in $2525: 81 336-219 X which the steel isfirst treated to produce a thin adherent 3088835 5/1963 Pirooz 106 53coating on the surface thereof, the improvement com 3106496 10/1963Anolick 2 prising, applying a glass coating to the surface of the3144364 8/1964 'gz' 148 113 treated steel, said glass coating having acoefiicient of 3200310 8/1965 Carman 54 thermal expansion lower thanthat of the steel, firing 3375144 3/1968 Ta lor 1' said coating,subjecting the steel to a predetermined 3407091 10/1968 g zg i X amountof tension in the rolling direction while the glass 3418710 12/1968seidell et a1 117 129 X coating on the steel is above its softeningpoint and cooling the glass coating below the softening point of theDEWAYNE RUTLEDGE, Primary Examiner glass while maintaining sald steel intenslon.

2. The method of claim 1 in which the glass exhibits a WHITE AsslstantExammer coetficient of expansion of less than 8 X10" in./in. C.

3. The method of claim 1 in which the steel is subjected ot a stress ofbetween 350 and 2000 pounds per 106'47, 53; 117-129; 14831.5, 31.55;336-219

